CTL: utility.h, use ctl::swap in string (#1227)

* Add ctl utility.h

Implements forward, move, swap, and declval. This commit also adds a def
for nullptr_t to cxx.inc. We need it now because the CTL headers stopped
including anything from libc++, so we no longer get their basic types.

* Use ctl::swap in string

The STL spec says that swap is located in the string_view header anyawy.
Performance-wise this is a noop, but it’s slightly cleaner.

ctl/utility.h

@@ -0,0 +1,61 @@
+// -*-mode:c++;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8-*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+#ifndef COSMOPOLITAN_CTL_UTILITY_H_
+#define COSMOPOLITAN_CTL_UTILITY_H_
+
+namespace ctl {
+
+namespace __ {
+
+template<typename T>
+struct no_infer_
+{
+    typedef T type;
+};
+
+template<typename T>
+using no_infer = typename no_infer_<T>::type;
+
+} // namespace __
+
+template<typename T>
+constexpr T&&
+move(T& t) noexcept
+{
+    return static_cast<T&&>(t);
+}
+
+template<typename T>
+constexpr T&&
+forward(__::no_infer<T>& t) noexcept
+{
+    return static_cast<T&&>(t);
+}
+
+template<typename T>
+// TODO(mrdomino): requires move_constructible<T> && move_assignable<T>
+constexpr void
+swap(T& a, T& b) noexcept
+{
+    T t(ctl::move(a));
+    a = ctl::move(b);
+    b = ctl::move(t);
+}
+
+template<typename T, size_t N>
+// TODO(mrdomino): requires is_swappable
+constexpr void
+swap(T (&a)[N], T (&b)[N]) noexcept
+{
+    for (size_t i = 0; i < N; ++i) {
+        swap(a[i], b[i]);
+    }
+}
+
+template<typename T>
+T&&
+declval() noexcept;
+
+} // namespace ctl
+
+#endif // COSMOPOLITAN_CTL_UTILITY_H_